1. Field of the Invention
The present invention relates to a hand-over technology in a mobile communication system.
2. Description of the Related Art
FIG. 7 shows an example of a mobile communication network (hereinafter, referred to as 3G network) standardized by the 3GPP (3rd Generation Partnership Project).
A mobile communication system 100 shown in FIG. 7 includes a core network 101 and a sub-network (Universal Terrestrial Radio Access Network (UTRAN) 102. The UTRAN 102 is composed of a plurality of Radio Network Systems (RNSs) 103-1 and 103-2.
Each of the RNSs 103-1 and 103-2 includes a radio network controller (RNC) 111 and a plurality of radio base stations (NodeBs) 112-1, 112-2 and others. A mobile terminal (i.e. user equipment) which is not shown in FIG. 7 changes a NodeB to connect with while moving. User equipment can continue a communication by applying the changing method. The changing method for a NodeB is called a hand-over.
At the time of a hand-over, the data which the user equipment cannot receive from a handover source radio base station (NodeB 112-1) is transmitted from the radio network controller (RNC) 111 to a hand-over target base station (NodeB 112-2). The user equipment can receive the data which is not received from the NodeB 112-1, from the NodeB 112-2. In a mobile communication system in which a soft hand-over technology is used, the RNC 111 simultaneously transmits data from both of the source radio base station (NodeB) 112-1 and a target radio base station (NodeB) 112-2 to the user equipment.
In a 3G network, the RNC 111 gives transmission timing to a radio base station (NodeB) for downlink data whose delay is limited. When it is expected that a delay exceeds a permitted delay value, even if a hand-over occurs, the RNC 111 does not transmit data to a hand-over target NodeB 112-2. In a 3G network, a RNC manages handling of downlink data in an integrated fashion. For this reason, in a 3G network, a delay of data can be kept below a permitted delay value.
In recent years, the 3GPP has been studying a next generation network (hereinafter, referred to as LTE/SAE network) under the name of Long Term Evolution (LTE) and System Architecture Evolution (SAE). The object of this study is to improve a throughput of user data, to reduce a call connection delay, to reduce a transmission delay of user data, to reduce a number of nodes and to reduce a number of interfaces to be standardized.
FIG. 1 shows an example of the LTE/SAE network. FIG. 1 is a figure also showing a configuration of an example of a mobile communication system according to the present invention. However, in a range shown in FIG. 1, the network is the same as the LTE/SAE network. Accordingly, in this specification, both of related arts and exemplary embodiments will be described with reference to FIG. 1.
Referring to FIG. 1, an LTE/SAE network 1 includes an Evolved Core Network (Evolved CN) 2 which is a core network and an Evolved Universal Terrestrial Radio Access Network (EUTRAN) 3 which is a sub-network. The EUTRAN 3 accommodates a plurality of nodes that include radio base stations (enhanced NodeB; eNodeB) 11-1 and 11-2. A mobile terminal, that is user equipment (UE) 4, is shown in FIG. 1. The UE communicates with eNodeBs 11-1 and 11-2.
When the user equipment 4 performs a hand-over between eNodeBs in the LTE/SAE network 1, it is considered to perform following data transmission. Suppose, for example, that a part of downlink data transmitted from the Evolved CN 2 to the source eNodeB 11-1 is not transmitted to user equipment. In such case, the hand-over source base station 11-1 transmits the data which is not transmitted to the hand-over target base stations 11-2. The hand-over target base station 11-2 transmits the transmittes data to the user equipment. In the LTE/SAE network 1, an occurrence of data that is not transmitted to user equipment during a hand-over is reduced.
In the LTE/SAE network 1 with such configuration, a delay value of downlink data is usually limited. A radio base station transmits data to user equipment so that a delay value of the data is equal or smaller than a permitted delay value. However, when a radio base station cannot transmit data to user equipment with a delay value which is equal or smaller than the permitted delay value, the radio base station performs a predetermined process to the data. The predetermined process includes disposal of the data.
JP2003-324761A describes that when a hand-over is performed, a hand-over source base station transmits a packet number or the like whose acknowledgement is not received to a hand-over target base station.
In JP2005-026941A, it is described that a mobile IP network system in which a packet within a packet transmission stop time is disposed. The packet transmission stop time is calculated based on a line speed, a line quality, the number of times that a hand-over is performed per unit time and traffic.
JP2001-128212A discloses that a radio network controller controls a timing of transmission to a radio terminal in a hand-over target base station based on a difference (DL1-DL2) between a transmission delay time (DL1) in a radio base station (A) and a transmission delay time (DL2) in a radio base station (B).
In FIG. 1, the time when user equipment 4 performs a hand-over from a radio base station (eNodeB) 11-1 to a radio base station (eNodeB) 11-2 is considered. Furthermore, at the time, suppose that data (including data whose acknowledgement from the user equipment 4 is not received by the eNodeB 11-1) which has not been transmitted to the user equipment 4, before the user equipment 4 disconnects the radio base station (eNodeB) 11-1, is remaining. In such case, the eNodeB 11-1 transmits the data that has not been transmitted, to the eNodeB 11-2. However, the eNodeB 11-2 cannot know a delay value, of the transmitted data, that is generated in the eNodeB 11-1. Moreover, the transmitted data delays in the eNodeB 11-2. As a result, a delay value of the transmitted data may exceed a permitted delay value. The eNodeB 11-2 cannot know the fact. Accordingly, the target eNodeB 11-2 cannot necessarily perform a predetermined process mentioned above.